1. Field of the Invention
The present invention relates to a technology for measuring aspheric surface profile of an optical element such as an aspheric lens.
2. Description of the Related Art
As a non-contact measuring method for fast measuring aspheric surface profile of an aspheric lens, a method has been proposed in Jahannes Pfund, Norbert Lindlein and Johannes Schwider, “NonNull testing of rotationally symmetric aspheres: a systematic error assessment” (App. Opt. 40 (2001) p. 439), which projects light having a spherical wavefront onto an aspheric surface as a measurement object surface through an optical system and measures a measurement light reflected by the measurement object surface by using a Shack-Hartmann sensor provided as a light-receiving sensor. This measuring method has an advantage of being able to measure profiles of various designed measurement object surfaces, as compared with an interferometer using a null lens disclosed in Japanese Patent Laid-Open No. 9-329427. Moreover, this measuring method also has an advantage that, as compared with a stitching interferometer disclosed in Japanese Patent Laid-Open No. 2004-125768 which moves a sample during measurement and a scanning interferometer disclosed in Japanese Patent No. 3971747, there is no need to use a stage and a length measuring device for moving the sample with high accuracy, and a complex analysis program.
In the method using the Shack-Hartmann sensor, proposed in Jahannes Pfund, Norbert Lindlein and Johannes Schwider, “NonNull testing of rotationally symmetric aspheres: a systematic error assessment” (App. Opt. 40 (2001) p. 439), the aspheric measurement object surface does not reflect the measurement light perpendicularly thereto and therefore a ray angle of the reflected measurement light from the measurement object surface is different from a ray angle of the measurement light reaching the measurement object surface. Consequently, the reflected measurement light entering the light-receiving sensor is not collimated, which is detected as a wavefront significantly different from a planer wavefront. Thus, the wavefront of the measurement light reflected by the measurement object surface, measured by the light-receiving sensor, does not directly show the profile of the measurement object surface, unlike a Fizeau interferometer.
Calculation of the profile of the measurement object surface from the measured wavefront requires a positional magnification (so-called distortion, and also called a coordinate magnification) that is a ratio of lateral coordinates of the sensor surface 8a and lateral coordinates of the measurement object surface, and an angular magnification that is a ratio of ray angles on the sensor surface and on the measurement object surface.
However, these positional magnification and angular magnification are not constant with respect to distance from an optical axis, that is, have distribution. The distribution changes sensitively, especially to error of curvature radius of a lens included in the optical system, error of position in an optical axis direction (so-called alignment error), spherical aberration and others. Therefore, calibration for the distribution is needed. Japanese Patent Laid-Open Nos. 2000-97663, 10-281736, 2006-133059 and 2009-180554 disclose calibration methods for the positional magnification distribution.
The calibration method disclosed in Japanese Patent Laid-Open Nos. 2000-97663, 10-281736 and 2006-133059 performs calibration of the positional magnification distribution by moving a measurement object surface by a known distance and detecting a change amount of a measured value by a light-receiving sensor with respect to the movement of the measurement object surface. Thus, the method not only requires a stage for moving the measurement object surface with high accuracy and a length measurement device for measuring the movement distance with high accuracy, but also has difficulty in accurate calibration of both the positional and angular magnification distributions.
Moreover, the calibration method disclosed in Japanese Patent Laid-Open No. 2009-180554 performs calibration of the positional magnification distribution by moving part of an optical system of an interferometer. However, the method performs the calibration by using radii of interference fringes on a light-receiving portion as an indicator, which has a problem that cannot accurately measure the radii of the interference fringes because their pitch is too small. Furthermore, the method has difficulty in accurate calibration of the angular magnification distribution.